A. Field of the Invention
The present invention relates to a method of manufacturing a lead frame.
B. Description of the Prior Art
Conventionally, a thin plate type of lead frame materials (usually a copper or nickel alloy) are formed into a lead frame shape by stamping or etching. After the lead frame is formed, overflow plating with a metal (e.g., nickel) is performed on the resulting structure, followed with another overflow plating using a palladium (Pd) or palladium-nickel (Pd-Ni) alloy, to obtain a complete lead frame.
The overflow plating method may broadly be defined into an electrolyte plating method or non-electrolyte plating method, the electrolyte method being more common. The electrolyte plating method involves forcibly precipitating metallic positive ions, which are dissolved in a solution, onto the surface of the lead frame material to be plated by negatively charging the lead frame.
The metallic positive ions receiving electrons are precipitated as nuclei on the surface of the lead frame. These nuclei are nonuniformly precipitated, depending on the shape of the lead frame, the current density and concentration distribution of the metallic positive ions dissolved in the solution. When the nucleus is generated at a position on the surface of the lead frame, the metal grows around the nucleus at a growth rate greater than that of the adjacent nucleus. This phenomenon is evident when uniform crystal line orientation of the plating layer is observed through an X-ray diffraction analysis.
In such a plating structure, gases or pores are led between the nuclei on the plating layer, and consequently corrosive ions, e.g., chloride ion (Cl.sup.-), can easily penetrate into the plating layer through the pores. In practice, if a salt spray test is performed on a lead frame after plating, which lead frame is composed of an alloy 42 (i.e., Ni-Fe alloy containing a 42% nickel), corrosion can be observed around the pore-formed partions on the entire surface of the lead frame within two to three hours. Such corrosion erodes the surface of the lead frame and lower the electrical conductivity of the lead frame, thus adversely affecting its characteristics catastrophically. Also, since palladium which is a conductive material has a high reactivity, a brittle fracture is caused by hydrogen ions induced from the plating solution, which weakens the plated surface.